The proposed studies focus on central nervous system (CNS) HIV infection and its consequences in treated patients with plasma viral suppression. In the first of two aims, we will use the ultrasensitive single copy assay (SCA) of viral RNA to define the presence and magnitude of cerebrospinal fluid (CSF) infection in a group of 'well-treated' patients and examine the impact of 'residual' CSF infection on CSF biomarkers of immunoactivation and neural injury and on neuropsychological test performance. We will also assess whether residual CSF infection relates to combination drug penetration characteristics. In the second aim, we will use sensitive single genome sequencing (SGS) methods and phylogenetic relationships to analyze the origins of viral escape and residual CSF HIV in treated patients. While antiretroviral therapy has had a remarkable impact on severe CNS HIV disease, persistent CNS infection in treated patients remains potentially important as the driver of chronic, continued CNS immunoactivation and injury, and as a viral reservoir that needs to be targeted by viral eradication strategies. The proposed studies will provide unique and essential information regarding the frequency, character and consequences of continued CNS HIV infection in 'well-treated' patients and also provide a foundation for eradication strategies targeting this infection.